High-molecular-weight polyethylene is a characteristic engineering plastic, which is excellent in terms of impact resistance and abrasion resistance, and has self-lubricating property, and thus, such high-molecular-weight polyethylene has been used in various fields. Since this high-molecular-weight polyethylene has a much higher molecular weight in comparison to commonly used polyethylene, it is anticipated that a molded product having high strength and high elasticity can be obtained by a high degree of orientation of the high-molecular-weight polyethylene. As such, various studies have been conducted to obtain such highly oriented high-molecular-weight polyethylene.
Patent Literature 1 discloses a technique regarding, what is called, a gel spinning process, in which gel-state fibers obtained by dissolving high-molecular-weight polyethylene in a solvent are stretched at a high ratio. Polyethylene fibers obtained by the gel spinning process have extremely high strength and high modulus of elasticity, and further, the fibers have been known to be extremely excellent in impact resistance. However, these polyethylene fibers have been problematic in terms of the aspects of environment, cost and production rate, since a large amount of solvent is used in the gel spinning process, a step of dissolving high-molecular-weight polyethylene in a solvent and a step of drying a product after completion of stretch molding are required, and the like.
On the other hand, a molding method of compressing high-molecular-weight ethylene polymer particles at a temperature lower than the melting point and then stretching them, namely, a so-called solid-phase stretching method, has also been developed. The solid-phase stretching method is considered to be superior to the gel spinning process in terms of a processing process in that no solvents are used. However, in the solid-phase stretching method, since polymer particles are compressed, rolled and stretched at the melting point or lower, insufficient pressure bonding of polymer particles, insufficient stretching caused by the entanglement of polymer chains, and the like have occurred, and thus, a problem of this method is that molding processing is very difficult.
In contrast, Patent Literatures 2 and 3 disclose a technique of achieving high molecular weight, high crystallinity, and regulation of particle surface structure, particle diameter and molecular weight distribution, so as to improve the processability of an ethylene polymer and the mechanical strength of a molded product. However, even in the case of a solid-phase stretch-molded product obtained from such an ethylene polymer, the mechanical strength of the solid-phase stretch-molded product has not yet exceeded the mechanical strength of a stretch-molded product obtained by the gel spinning process.
As a method of regulating a higher-order structure to obtain an ethylene polymer suitable for solid-phase stretching, and in particular, as a method of reducing entanglement in polymers, a method of suppressing the interference of polymer molecular chains in polymerization with other polymer molecular chains has been considered. Patent Literature 4 and Non Patent Literature 1 disclose a technique of utilizing, as polymerization fields, the pores of mesoporous silica used as a polymerization catalyst, to suppress free movements of molecules during the growth of the polymer, so as to suppress the entanglement of molecular chains and control crystallinity and the morphology of the obtained polymer.